Results of preliminary studies on the guinea pig ovary indicate the presence of ovarian cysts which arise spontaneously and with a high degree of frequency. Morphologically these cysts are similar to the benign serous cystadenoma of human ovaries. The purpose of the proposed study is morphologically to define such cysts and stages of their development as they arise in the ovaries of Hartley strain guniea pigs. Ovaries from animals of several ages will be examined by light and electron microscopy by histochemistry and by morphometry. Information will be sought on changes in tissue and cell structure and permeability, in membrane structure and staining characteristics, and in enzymatic activity related to secretion and transport. The methodologies involved will include scanning and transmission electron microscopy, freeze fracture replication, horseradish peroxidase and lanthanum tracing, lectin staining and electron microscopic demonstration of phosphatase activities. The information derived will be applied to future investigations of the cause, control, and potential for malignant transformation of this cyst.